Because there are so many stitches on your cast on, she probably recommended the circular needles for the length. Just knit across using one tip to knit into the other tip. Then turn the whole thing around to knit back the other way. You'll have to slide your work from one end of the needle to the other to get at the first stitch of the next row.
The main purpose of circs is to knit things round and round, but I don't think this is the case here.
And welcome! :D
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry